The present invention relates to optical compensation films applied to a twisted nematic liquid crystal display. This compensation film improves viewing angle, especially in the vertical direction. It also has a low average tilt angle, and large retardation value and thickness, therefore allows an easy manufacturing.
The following terms have the definitions as stated below.
Optic axis herein refers to the direction in which propagating light does not see birefringence.
A-plate, C-plate and O-plate herein are the plates in which the optic axis is in the plane of the plate, perpendicular to the plate surface and tilted with respect to the plane of the plate, respectively.
Polarizer and Analyzer herein refer to elements that polarize electromagnetic wave. One closer to the source of the light is generally called polarizer while the one closer to the viewer is called analyzer.
Viewing direction herein is defined as a set of polar viewing angle xcex1 and azimuthal viewing angle xcex2 as shown in FIG. 1 with respect to a liquid crystal display 101. The polar viewing angle xcex1 is measured from display normal direction 103 and the azimuthal viewing angle xcex2 spans between an appropriate reference direction 105 in the plane of the display surface 107 and the projection 108 of the arrow 109 onto the display surface 107. Various display image properties, such as contrast ratio, color and brightness are functions of angles xcex1 and xcex2.
Azimuthal angle xcex8 and tilt angle xcfx86 are herein used to specify the direction of optic axis. For the transmission axes of the polarizer and the analyzer, only the azimuthal angle is used as their tilt angle is zero. FIG. 2 shows the definition of the azimuthal angle xcfx86 and tilt angle xcex8 to specify the direction of optic axis 201 with respect to the x-y-z coordinate system 203. The x-y plane is parallel to the display surface 107, and the z-axis is parallel to the display normal direction 103. The azimuthal angle xcfx86 is the angle between the x-axis and the projection of the optic axis 201 onto the x-y plane. The tilt angle xcex8 is the angle between the optic axis 201 and the x-y plane.
ON (OFF) state herein refers to the state with (without) an applied electric field to the liquid crystal display 101.
Isocontrast plot herein shows a change in a contrast ratio from different viewing directions. Isocontrast line, on which the contrast ratio is constant (such as 10, 50 and 100), is plotted in polar format. The concentric circle corresponds to polar viewing angle xcex1=20xc2x0, 40xc2x0, 60xc2x0 and 80xc2x0 (outer most circle) and the radial lines indicates azimuthal viewing angle xcex2=0xc2x0, 45xc2x0, 90xc2x0, 135xc2x0, 180xc2x0, 225xc2x0, 270xc2x0 and 315xc2x0.
Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizer and analyzer. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the analyzer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, liquid crystal technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display (LCD) is the propensity for light to xe2x80x9cleakxe2x80x9d through liquid crystal elements or cells, which are in the dark or xe2x80x9cblackxe2x80x9d pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about the normal incidence (xcex1=0xc2x0) to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or house hold appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, LCD""s application as a replacement of the traditional CRT will be limited.
Among various modes of LCD, Twisted Nematic (TN) LCD is the most prevalent ones. FIG. 3A is a schematic of a TN-LCD 313. A liquid crystal cell 301 is positioned between polarizer 303 and analyzer 305. Their transmission axes 307, 309 are crossed, meaning that the transmission (or equivalently, absorption) axes of polarizer and analyzer form angle 90xc2x110xc2x0. Inside the liquid crystal cell, optic axis of liquid crystal shows azimuthal rotation of 90xc2x0 in the OFF state across the cell thickness direction. In FIG. 3A, the direction of liquid crystal optic axis 311 at the cell surfaces is indicated by a single head arrow. At the surface, the liquid crystal optic axes 311 form small tilt angle xcex8s with respect to the cell surfaces in order to prevent the reverse twist. Namely, the tilt is consistent with the sense (clock or counter-clock wise) of the azimuthal rotation in the liquid crystal optic axis in the cell thickness direction. Un-polarized incoming light is linearly polarized by the polarizer 303 and its plane of polarization rotates 90xc2x0 while traveling through the liquid crystal cell 301. The plane of polarization of out-coming light from the cell 301 is parallel to the transmission axis 309 of analyzer 305 and will transmit through the analyzer 305. With sufficiently high-applied voltage, the liquid crystal becomes perpendicular to the cell plane except in the close vicinity of the bounding plates. In this ON state, the incoming polarized light essentially does not see birefringence and thus it is blocked by the analyzer. This mode (bright OFF state and dark ON state) is called Normally White Twisted Nematic Liquid Crystal Display (NW-TN-LCD) mode.
In the display normal viewing direction (xcex1=0xc2x0), one can obtain high contrast between the ON and OFF states. However, when the display is viewed from an oblique direction, the propagating light sees birefringence in ON state, thus it is not completely blocked by the analyzer. This light leakage gives low contrast ratio in the oblique direction. As is well known in the art, the degradation in the contrast ratio due to the leakage is more significant in the vertical viewing direction (azimuthal angle xcex2=90xc2x0, 270xc2x0) than it is in the horizontal direction (xcex2=0xc2x0, 180xc2x0) in TN-LCD. The azimuthal angle xcex2 is measured from a reference direction 205 (shown in FIG. 1), which is chosen to be at 45xc2x0 relative to the transmission axes 307, 309 of the polarizer 303 and analyzer 305. The isocontrast plot of the display 313 is shown in FIG. 3B. The lines 315, 317, 319 are isocontrast ratio lines for contrast ratios 10, 50 and 100, respectively. The display fails to maintain contrast ratio 10 or higher in the range required for many applications. The contrast ratio quickly deteriorates as the viewing direction deviates from the normal direction. This deterioration is particularly significant in the upper vertical viewing direction (xcex2=90xc2x0).
One of the common methods to improve the viewing angle characteristic of TN-LCD is to use the compensation films. In some cases, the compensation films consist of optically anisotropic layer deposited on the substrate. The substrate can be flexible film such as triacetyl-cellulose or solid such as glass. The optically anisotropic layer is generally made of liquid crystal polymers. As it is necessary to align the optic axis of the liquid crystal polymer in the desired direction (making the anisotropic layer as A, C or O-plate), an alignment layer is often deposited between the optically anisotropic layer and substrate or between the two optically anisotropic layers. The thickness of the anisotropic layer depends on the property of its constituent material and LCD to which it is applied. The compensation films are typically inserted anywhere between the liquid crystal cell and the polarizers. The function of compensation film, in general, is to undo the phase retardation experienced by the propagating light while traveling through the liquid crystal cell. By using the compensation film in the ON state of NW-TN-LCD, birefringence experienced by the obliquely propagating light is cancelled by the film. This gives us uniform dark state resulting in improved viewing angle characteristic.
Various compensation methods have been suggested. U.S. Pat. No. 5,619,352 discloses the usage of combinations of O-plates. The basic idea is to compensate the ON state of TN-LCD by a stack of O-plate that has similar or complementary optical symmetries of ON state liquid crystal cell. This was done by approximating the ON state of TN-LCD by three representative parts: two regions closed to the cell bounding plates and cell middle section. In the middle of the cell, the liquid crystal optic axis is almost perpendicular to the cell plane with large change in the azimuthal angle of liquid crystal optic axis. In the vicinity of the bounding plates, tilt in liquid crystal optic axis is small with almost fixed azimuthal angle. In comparison to the previous compensation technology, the stability of gay scale as well as viewing angle characteristic has been improved.
Chen et al. suggested (SID99, pp.98-101 xe2x80x9cWide-viewing-angle photo-aligned plastic films for TN-LCDsxe2x80x9d) the use of crossed O-plate to compensate the dark state of NW-TN-LCD. The O-plate with tilt angle xcex8=20xc2x0 is used. FIG. 4A shows structures of TN-LCD 441 with O-plates 403 (also referred as optically anisotropic layer). In addition to the liquid crystal cell 401, four O-plates are placed between crossed polarizer 411 and analyzer 413. The optic axes 415 in the O-plates tilt 20xc2x0 with respect to the plate plane. The azimuthal angles of optic axes and transmission axes of analyzer and polarizer are given in the parenthesis. The azimuthal angle of O-plate is equal to that of the transmission axes 423, 425 of the polarizer 411 or the analyzer 413. FIG. 4B shows the isocontrast plot of the display 441. The lines 427, 429 and 431 are isocontrast lines of contrast ratios 10, 50 and 100 respectively. With the O-plate compensation, the viewing angle characteristic was improved. The area with contrast ratio 10 or higher expanded significantly compared to FIG. 3B.
The prior art compensators using O-plate improved the viewing angle characteristic by reducing the light leakage in the ON state. However, viewing angle characteristic in the vertical viewing direction is far from satisfactory. For example, contrast ratio becomes less than 10 for wide viewing angle range: 60xc2x0xe2x89xa6xcex2xe2x89xa6120xc2x0 and 50xc2x0xe2x89xa6xcex1 with the display 441 shown in FIG. 4A. This low viewing angle characteristic in wide range limits the applications of LCD. Prior arts call for a tilt angle xcex8 equal to or higher than 20xc2x0. It is well known in the art, however, that the repeatability in generation of high tilt angle in the manufacturing process is limited. The prior art also requires a small phase retardation value of 60 nm from the compensation film. If the birefringence of the material is 0.1, the thickness of one optically anisotropic layer would be 0.6 xcexcm. However, from the production point of view, the uniform coating of anisotropic layer on the substrate is more of a technical difficulty if the coating thickness is smaller. As a result, the prior art O-plate compensation requiring sub-micron thickness makes the uniform coating extremely difficult. Therefore there is a strong need for new compensation film that offers better viewing angle characteristic with smaller tilt angle and larger coating thickness in optically anisotropic layers.
This compensation film improves viewing angle of twisted nematic liquid crystal display, especially in the vertical viewing direction. It also has a low average tilt angle, and large retardation value and thickness, therefore allows an easy manufacturing.
It is one of our objectives to provide a compensation film, which can be used in conjunction with a twisted nematic liquid crystal display for an improved viewing angle characteristic, especially in the vertical viewing direction.
It is another of our objectives to provide a compensation film, which has a large thickness, and therefore can be fabricated with a simpler process.
It is yet another of our objectives to provide a compensation film, which has a low average tilt angle, and therefore increases repeatability of the performance of the film.